Nonwoven web with bimodal fiber distribution

ABSTRACT

A nonwoven web comprising a layer of polymeric fibers, wherein, based on the total number of polymeric fibers, at least 10% the polymeric fibers in said layer are coarse fibers having a fiber diameter of 4 μm or more, and at least 10% of the polymeric fibers in said layer are fine fibers having a fiber diameter of 2 μm or less. Also described herein is a method for making the nonwoven web, comprising melt-blowing a polymer mixture comprising two immiscible or partially miscible polymers.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication No. 62/044,629 filed Sep. 2, 2014, the content of which isincorporated herein by reference in its entirety.

BACKGROUND

Current melt blown technology produces uniform fibers that may be eithercoarse or fine fibers, but cannot make both coarse and finer fiberssimultaneously without multiple layers. In many applications, each rangeof fiber diameters has desirable properties.

SUMMARY

Described herein a melt blown media with distinct bimodal fiberdistribution resulting in coarse structural fibers for low pressure dropand fine fibers for high efficiency within one single melt blown layer.

Various embodiments described herein relate to a nonwoven web comprisinga layer of polymeric fibers, wherein, based on the total number ofpolymeric fibers, at least 10% the polymeric fibers in said layer arecoarse fibers having a fiber diameter of 4 μm or more, and at least 10%of the polymeric fibers in said layer are fine fibers having a fiberdiameter of 2 μm or less. In some embodiments, the polymeric fibers inthe nonwoven web have bimodal distribution of their fiber diameters.

Further embodiments described herein relate to a method for making thenonwoven web, comprising melt-blowing a polymer mixture comprising atleast two immiscible or partially miscible polymers. In someembodiments, the polymer mixt, on oh come on you piece of shit urecomprises liquid crystalline polymer (LCP) and polybutyleneterephthalate (PBT). In some embodiments, the polymer mixture is made bycompounding and/or blending at least two polymers.

These and other features, together with the organization and manner ofoperation thereof, will become apparent from the following detaileddescription when taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows example SEM images of melt blown fibers from (a) standardPBT (×1000), (b) 5% LCP in PBT (×1000), and (c) 2.5% LCP in PBT (×1000),and (d) 2.5% LCP in PBT (×500).

FIG. 2 shows example bimodal fiber diameter distribution of 2.5% LCPsample.

FIG. 3 shows example SEM images (×5000) of sub-micron fibers on the meltblown media of 2.5% LCP in PBT.

FIG. 4 shows example fiber size distribution comparisons betweenexisting media and the inventive nonwoven web described herein.

FIG. 5 shows example performance comparison between an existing productand the inventive nonwoven web described herein (the melt blown media of2.5% LCP in PBT).

DETAILED DESCRIPTION

A bimodal distribution of fiber diameters possesses distinct advantagesover other single layers and even gradient fiber structures of meltblown. A method of compounding and blending polymers of significantlydifferent rheological properties results in a melt blown layer with acontrolled bimodal fiber distribution. The fine fiber population in thedistribution is composed of a significant portion of sub-micron fiberswhich are even finer than normal melt blowing allows. The coarse fiberscontribute higher permeability, while the very fine fibers results ineven higher efficiency than uniform or gradient media of similarpermeability.

Controlled processing of immiscible or partially miscible polymers, suchas LCP and PBT, results in unique transitions from coarse to fine fiber.The LCP described herein refers to any liquid crystalline polymer ofaromatic polyester with a highly ordered structure in both melt andsolid states. Preparation of LCP/PBT materials with designed mixingratios by using polymer compounding, physical blending, or combinationof compounding and blending, and controlled melt blowing processconditions result in a LCP/PBT media with wider and unique bimodal fiberdistribution and improved performance.

This LCP/PBT media can be used in any current air or liquid filtrationor coalescing system currently using PBT with equivalent temperature,chemical, and hydrolytic resistance. Trials completed on pilot machinedemonstrate ability for these materials to run on production machines,with the unique and advantageous properties for applications in fuel,lube, crankcase ventilation (CV), and air filtration products. Thecoarse structure leads to high permeability yet the finer fiber contentcontributes high efficiency.

The nonwoven web described herein may comprise, for example, a μm ofpolymeric fibers, wherein, based on the total number of polymericfibers, at least 10% of the fibers in said layer are coarse fibershaving a fiber diameter of 3 μm or more, or 4 μm or more, or 5 μm ormore, and at least 10% of the polymeric fibers in said layer are finefibers having a fiber diameter of 3 μm or less, or 2 μm or less, or 1 μmor less.

In some embodiments of the nonwoven web, at least 10%, or at least 20%,or at least 30%, or about 30-60% of the polymeric fibers are coarsefibers having a fiber diameter of 3 μm or more, based on the totalnumber of polymeric fibers. In some embodiments of the nonwoven web, atleast 10%, or at least 20%, or at least 30%, or about 30-60% of thepolymeric fibers are coarse fibers having a fiber diameter of 4 μm ormore, based on the total number of polymeric fibers. In some embodimentsof the nonwoven web, at least 10%, or at least 20%, or at least 30%, orabout 30-60% of the polymeric fibers are coarse fibers having a fiberdiameter of 5 μm or more, based on the total number of polymeric fibers.

In some embodiments of the nonwoven web, at least 10%, or at least 20%,or at least 30%, or at least 40%, or about 40-70% of the polymericfibers are fine fibers having a fiber diameter of 3 μm or less, based onthe total number of polymeric fibers. In some embodiments of thenonwoven web, at least 10%, or at least 20%, or at least 30%, or atleast 40%, or about 40-70% of the polymeric fibers are fine fibershaving a fiber diameter of 2 μm or less, based on the total number ofpolymeric fibers. In some embodiments of the nonwoven web, at least 10%,or at least 20%, or at least 30%, or at least 40%, or about 40-70% ofthe polymeric fibers are fine fibers having a fiber diameter of 1 μm orless, based on the total number of polymeric fibers.

In some embodiments of the nonwoven web, the polymeric fibers havebimodal distribution of their fiber diameters. The bimodal distributioncan comprise a first distribution population and a second distributionpopulation, as shown in FIG. 3.

The first distribution population (d₁) can comprise about 40-70% of thetotal number of polymeric fibers, with a first distribution peak at afiber diameter of 3 μm or less, or 2 μm or less, or 1 μm or less. Thegeometric mean diameter of the first distribution population can beabout 0.50-2 μm or, more specifically, 0.75-2 μm.

The second distribution population (d₂) can comprise about 30-60% of thetotal number of polymeric fibers, with a second distribution peak at afiber diameter of 3 μm or more, or 4 μm or more, or 5 μm or more, or 6μm or more. The geometric mean diameter of the second distributionpopulation can be about 3-15 μm.

The first distribution population and the second distribution populationcan be separated by a distribution trough, with a minimum distributionpoint (d_(m)) at a fiber diameter of 1.5-7 μm. In some embodiment, 20%or less, or 10% or less, or 5% or less of the total number of polymericfibers have a fiber diameter within ±0.5 μm of d_(m). In other words,the fiber population of (d_(m)−0.5 μm, d_(m)+0.5 μm) can be less than20%, or less than 10%, or less than 5% of total fiber amount.

In some embodiments of the nonwoven web, the polymeric fibers aremelt-blown fibers. The polymeric fibers can comprise, for example, twoimmiscible or partially miscible polymers. The polymeric fibers cancomprise, for example, 0.5-10 wt. % LCP. The polymeric fibers cancomprise, for example, PBT. The polymeric fibers can comprise 0.5-10 wt.% LCP, with the remaining polymeric material within the fibers beingPBT. The LCP and PBT fibers are not necessarily separate but can be inthe form of a polymeric mixture, mixed by weight % in the recipe.

In some embodiments, the nonwoven web has an air permeability of 20 cfmor more, or 40 cfm or more, or 50 cfm or more, or 60 cfm or more, or 70cfm or more, or between 40-60 cfm, or between 50-70 cfm, or between60-80 cfm, or between 70-90 cfm. Media permeability was measured perstandard textile method INDA IST 70.1 or ASTM D737-96 with a sample sizeof 38.3 cm² to a differential pressure of 125 Pa.

In some embodiments, the nonwoven web has a filtration ratio at particlesize of 10 μm (or 10 μm efficiency) of 40% or more, or 60% or more, or70% or more, or 80% or more, or 90% or more.

In some embodiments, the nonwoven web has a filtration capacity of 130mg/in² or more, or 140 mg/in² or more, or 150 mg/in² or more, or 160mg/in² or more, or 170 mg/in² or more, or 180 mg/in² or more. Mediaefficiency and capacity data were collected on a multipass test standusing the ISO 4548-12 standard. For these tests the sample size was 200cm² with a terminal pressure drop of 240-250 kPa and a test flow rate of3 liters per minute. In other embodiments, the nonwoven web may have alower filtration capacity, for example of 70 mg/in² or even 50 mg/in².

The nonwoven web described here can be made by, for example,melt-blowing a polymer mixture comprising two immiscible or partiallymiscible polymers.

In some embodiments, the polymer mixture is obtained by compounding thetwo immiscible or partially miscible polymers. Alternatively, thepolymer mixture may be obtained by physically blending the twoimmiscible or partially miscible polymers. Still further, the polymermixture is obtained by both compounding and physically blending.

In some embodiments, the polymer mixture comprises 0.5-30 wt. % LCP, andPBT.

In some embodiments, the polymer mixture is melt-blown according to thefollowing process conditions on a 0.5 meter wide pilot line: (a) dietemperature of 250-310° C., (b) air flow rate of 4-14 m³/min, (c)throughput of 4-20 kg/hr, (d) drum collector distance of 10-50 cm, and(e) extruder speed of 50-120 rpm.

ADDITIONAL EMBODIMENTS Embodiment 1

A nonwoven web comprising a layer of polymeric fibers, wherein, based onthe total number of polymeric fibers, at least 10% of the polymericfibers in said layer are coarse fibers having a fiber diameter of 4 μmor more. At least 10% of the polymeric fibers in said layer are finefibers having a fiber diameter of 2 μm or less.

Embodiment 2

The nonwoven web of Embodiment 1, wherein at least 25% polymeric fibersin said layer are coarse fibers having a fiber diameter of 4 μm or more,based on the total number of polymeric fibers.

Embodiment 3

The nonwoven web of any of Embodiments 1-2, wherein at least 25%polymeric fibers in said layer are coarse fibers having a fiber diameterof 5 μm or more, based on the total number of polymeric fibers.

Embodiment 4

The nonwoven web of any of Embodiments 1-3, wherein at least 25%polymeric fibers in said layer are fine fibers having a fiber diameterof 2 μm or less, based on the total number of polymeric fibers.

Embodiment 5

The nonwoven web of any of Embodiments 1-4, wherein at least 25%polymeric fibers in said layer are fine fibers having a fiber diameterof 1 μm or less, based on the total number of polymeric fibers.

Embodiment 6

The nonwoven web of any of Embodiments 1-5, wherein the polymeric fibersin said layer have bimodal distribution of their fiber diameters, basedon the total number of polymeric fibers.

Embodiment 7

The nonwoven web of any of Embodiments 1-6, wherein the polymeric fibersin said layer have bimodal distribution of their fiber diameters,comprising a first peak at a first fiber diameter of 2 μm or less and asecond peak at a second fiber diameter of 4 μm or more.

Embodiment 8

The nonwoven web of any of Embodiments 1-7, wherein the polymeric fibersin said layer have bimodal distribution of their fiber diameters,comprising a first peak at a first fiber diameter of 2 μm or less, asecond peak at a second fiber diameter of 4 μm or more, and a trough ata third fiber diameter between the first fiber diameter and the secondfiber diameter, wherein 10% or less of the polymeric fibers have a fiberdiameter within ±0.5 μm of the third fiber diameter, based on the totalnumber of polymeric fibers.

Embodiment 9

The nonwoven web of any of Embodiments 1-8, wherein the polymeric fibersin said layer have bimodal distribution of their fiber mean, comprisinga first peak at a first fiber diameter of 2 μm or less, a second peak ata second fiber diameter of 4 μm or more, and a trough at a third fiberdiameter between the first fiber diameter and the second fiber diameter,wherein 5% or less of the polymeric fibers have a fiber diameter within±0.5 μm of the third fiber diameter, based on the total number ofpolymeric fibers.

Embodiment 10

A nonwoven web comprising a layer of polymeric fibers, wherein, based onthe total number of polymeric fibers, at least 10% of the polymericfibers in said layer are coarse fibers having a fiber diameter of 3 μmor more, and at least 10% of the polymeric fibers in said layer are finefibers having a fiber diameter of 1 μm or less.

Embodiment 11

The nonwoven web of Embodiment 10, wherein at least 25% polymeric fibersin said layer are coarse fibers having a fiber diameter of 3 μm or more,and at least 25% polymeric fibers in said layer are fine fibers having afiber diameter of 1 μm or less, based on the total number of polymericfibers.

Embodiment 12

The nonwoven web of Embodiment 10 or 11, wherein the polymeric fibers insaid layer have bimodal distribution of their fiber diameters,comprising a first peak at a first fiber diameter of 1 μm or less, asecond peak at a second fiber diameter of 3 μm or more, and a trough ata third fiber diameter between the first fiber diameter and the secondfiber diameter, wherein 10% or less of the polymeric fibers have a fiberdiameter within ±0.5 μm of the third fiber diameter, based on the totalnumber of polymeric fibers.

Embodiment 13

A nonwoven web comprising a layer of polymeric fibers, wherein, based onthe total number of polymeric fibers, at least 10% of the polymericfibers in said layer are coarse fibers having a fiber diameter of 5 μmor more, and at least 10% of the polymeric fibers in said layer are finefibers having a fiber diameter of 3 μm or less.

Embodiment 14

The nonwoven web of Embodiment 13, wherein at least 25% polymeric fibersin said layer are coarse fibers having a fiber diameter of 5 μm or more,and at least 25% polymeric fibers in said layer are fine fibers having afiber diameter of 3 μm or less, based on the total number of polymericfibers.

Embodiment 15

The nonwoven web of Embodiment 13 or 14, wherein the polymeric fibers insaid layer have bimodal distribution of their fiber diameters,comprising a first peak at a first fiber diameter of 3 μm or less, asecond peak at a second fiber diameter of 5 μm or more, and a trough ata third fiber diameter between the first fiber diameter and the secondfiber diameter, wherein 10% or less of the polymeric fibers have a fiberdiameter within ±0.5 μm of the third fiber diameter, based on the totalnumber of polymeric fibers.

Embodiment 16

The nonwoven web of any of Embodiments 1-15, wherein the polymericfibers are melt-blown fibers.

Embodiment 17

The nonwoven web of any of Embodiments 1-16, wherein the polymericfibers comprises 0.5-10 wt. % liquid crystalline polymer.

Embodiment 18

The nonwoven web of any of Embodiments 1-17, wherein the polymericfibers comprises polybutylene terephthalate.

Embodiment 19

The nonwoven web of any of Embodiments 1-18, wherein the nonwoven webhas an air permeability of 40 cfm or more.

Embodiment 20

The nonwoven web of any of Embodiments 1-19, wherein the nonwoven webhas a filtration ratio of 60% or more at particle size of 10 μm.

Embodiment 21

The nonwoven web of any of Embodiments 1-20, wherein the nonwoven webhas a filtration capacity of 130 mg/in² or more.

Embodiment 22

A method for making the nonwoven web of any of Embodiments 1-21,comprising melt-blowing a polymer mixture comprising at least twoimmiscible or partially miscible polymers.

Embodiment 23

The method of Embodiment 22, wherein the polymer mixture is obtained bycompounding and/or physically blending the two immiscible or partiallymiscible polymers.

Embodiment 24

The method of Embodiment 22 or 23, wherein the polymer mixture comprises(a) 0.5-30 wt. % liquid crystalline polymer and (b) polybutyleneterephthalate.

Embodiment 25

The nonwoven web of any of Embodiments 1-21, wherein said layer ofpolymeric fibers is obtained by a method comprising melt-blowing apolymer mixture comprising at least two immiscible or partially misciblepolymers, wherein the polymer mixture is obtained by compounding and/orphysically blending the two immiscible or partially miscible polymers.

Embodiment 26

The nonwoven web of any of Embodiments 1-21, wherein said layer ofpolymeric fibers is obtained by a method comprising melt-blowing apolymer mixture comprising (a) 0.5-30 wt. % liquid crystalline polymerand (b) polybutylene terephthalate, wherein the polymer mixture isobtained by compounding and/or physically blending the liquidcrystalline polymer and the polybutylene terephthalate.

WORKING EXAMPLES

Material Preparation.

Polymer mixture comprising PBT and 0.5-10 wt. % LCP were made accordingto one of the following three options: (1) compounding LCP and PBT; (2)physically blending LCP and PBT; and (3) combination of compounding andblending (e.g., use compounded LCP/PBT alloy comprising 0-40 wt. % LCPto physically blend with neat PBT (1-30 wt. % alloy) to finally get0.5-10 wt. % LCP at finish).

Melt Blown Process.

The polymer mixtures prepared as described above were melt-blownaccording to the following process conditions on a 0.5 meter wide pilotline: (a) die temperature: 250-310° C.; (b) air flow rate: 4-14 m³/min;(c) throughput: 4-20 kg/hr; (d) drum collector distance: 10-50 cm; (e)extruder speed: 50-120 rpm. Nonwoven webs comprising both coarse fibersand fine fibers in a single layer were obtained, as shown in FIGS. 1 and3.

Fiber Diameter Distribution of Nonwoven Web.

As shown in FIGS. 2 and 4, the nonwoven web has wider and bimodal fiberdiameter distribution and includes sub-micron, fine micron, and coarsemicron fibers in one melt blown layer, which could lead to morecomprehensive performance than any one of them. Statistical analysisshows that the fiber diameter distribution of the nonwoven web comprises(i) a first distribution population (0<d₁<d_(m)) having a geometric meandiameter GMd₁ of 0.75-2 μm and representing 40%-70% of total fiberamount; (ii) a second distribution population (d₂>d_(m)) having ageometric mean diameter GMd₂ of 3-15 μm and representing 30%-60% oftotal fiber amount, wherein GMd₂=(4-7.5)×GMd₁; and (iii) a minimumdistribution point (d_(m)) between first and second distributionpopulations, wherein d_(m)=1.5-7 μm and d_(m)=(2-3.5)×GMd₁, and whereinthe population of (d_(m)−0.5 μm, d_(m)+0.5 μm) represents less than 5%of total fiber amount.

Performance of Nonwoven Web.

As shown in FIG. 5, the bimodal media has lower basis weight and betterperformance than a current melt blown polymer lube filter media. Coarserfibers in LCP/PBT media can lead to higher air permeability (lowerrestriction), while finer fibers can still provide very good efficiency.The media property and performance can be well controlled by processingrecipe. In addition, single layer process and low air flow rate in meltblowing process could result in energy saving. Aerosol applications,such as crankcase ventilation (CV), are likely to benefit.

LCP/PBT materials were prepared by polymer compounding, physicalblending, and combination of compounding and blending with designedcompositions. LCP and PBT polymers have significantly differentrheological and thermal properties. Without modifying the die,controlled melt blown processing of this immiscible LCP/PBT systemresulted in unique structure containing both coarse and fine fiberswhich were formed simultaneously in single layer media process.

As a result, this melt blown LCP/PBT media has much wider fiber sizedistribution, ranging from 0.2 μm to 30 μm, than normal melt blownpolyester media does. As shown in FIG. 3, the fine fiber population inthe distribution is composed of a significant portion of sub-micronfibers which are even finer than normal melt blowing allows. The coarsefibers contribute higher permeability while the very fine fibers resultsin even higher efficiency than uniform or gradient media of similarpermeability. This LCP/PBT media may be used in any current air orliquid filtration or coalescing system currently using PBT withequivalent temperature, chemical, and hydrolytic resistance. The widerand unique bimodal distribution of fiber diameters has distinctadvantages over other single layers and even gradient fiber structuresof melt blown media.

As used herein, the singular terms “a,” “an,” and “the” include pluralreferents unless the context clearly dictates otherwise. Thus, forexample, reference to a fiber can include multiple fibers unless thecontext clearly dictates otherwise.

As used herein, the term “fiber diameter” is used to describe theaverage or mean diameter of a particular fiber.

As used herein, the terms “substantially” and “about” are used todescribe and account for small variations. When used in conjunction withan event or circumstance, the terms can refer to instances in which theevent or circumstance occurs precisely as well as instances in which theevent or circumstance occurs to a close approximation. For example, theterms can refer to less than or equal to ±10%, such as less than orequal to ±5%, less than or equal to ±4%, less than or equal to ±3%, lessthan or equal to ±2%, less than or equal to ±1%, less than or equal to±0.5%, less than or equal to ±0.1%, or less than or equal to ±0.05%.

Additionally, amounts, ratios, and other numerical values are sometimespresented herein in a range format. It is to be understood that suchrange format is used for convenience and brevity and should beunderstood flexibly to include numerical values explicitly specified aslimits of a range, but also to include all individual numerical valuesor sub-ranges encompassed within that range as if each numerical valueand sub-range is explicitly specified. For example, a ratio in the rangeof about 1 to about 200 should be understood to include the explicitlyrecited limits of about 1 and about 200, but also to include individualratios such as about 2, about 3, and about 4, and sub-ranges such asabout 10 to about 50, about 20 to about 100, and so forth.

In the foregoing description, it will be readily apparent to one skilledin the art that varying substitutions and modifications may be made tothe invention disclosed herein without departing from the scope andspirit of the invention. The invention illustratively described hereinsuitably may be practiced in the absence of any element or elements,limitation or limitations, which is not specifically disclosed herein.The terms and expressions which have been employed are used as terms ofdescription and not of limitation, and there is no intention that in theuse of such terms and expressions of excluding any equivalents of thefeatures shown and described or portions thereof, but it is recognizedthat various modifications are possible within the scope of theinvention. Additionally, features from particular embodiments may becombined with features from other embodiments as would be understood byone of ordinary skill in the art. Thus, it should be understood thatalthough the present invention has been illustrated by specificembodiments and optional features, modification and/or variation of theconcepts herein disclosed may be resorted to by those skilled in theart, and that such modifications and variations are considered to bewithin the scope of this invention.

1. A nonwoven web comprising a layer of polymeric fibers, wherein, basedon a total number of polymeric fibers, at least 25% of the polymericfibers in said layer are coarse fibers having a fiber diameter of 4 μmor more, and at least 25% of the polymeric fibers in said layer are finefibers having a fiber diameter of 2 μm or less, and wherein thepolymeric fibers in said layer have a bimodal distribution of theirfiber diameters, comprising a first peak at a first fiber diameter of2μm or less, a second peak at a second fiber diameter of 4 μm or more,and a trough at a third fiber diameter between the first fiber diameterand the second fiber diameter, wherein 10% or less of the polymericfibers have a fiber diameter within ±0.5 μm of the third fiber diameter,based on the total number of polymeric fibers.
 2. (canceled)
 3. Thenonwoven web of claim 1, wherein at least 25% polymeric fibers in saidlayer are coarse fibers having a fiber diameter of 5 μm or more, basedon the total number of polymeric fibers.
 4. (canceled)
 5. The nonwovenweb of claim 1, wherein at least 25% polymeric fibers in said layer arefine fibers having a fiber diameter of 1 μm or less, based on the totalnumber of polymeric fibers.
 6. The nonwoven web of claim 1, wherein thepolymeric fibers in said layer have a bimodal distribution of theirfiber diameters, based on the total number of polymeric fibers.
 7. Thenonwoven web of claim 1, wherein the polymeric fibers in said layer havea bimodal distribution of their fiber diameters, comprising a first peakat a first fiber diameter of 2 μm or less and a second peak at a secondfiber diameter of 4 μm or more.
 8. (canceled)
 9. The nonwoven web ofclaim 1, wherein the polymeric fibers in said layer have a bimodaldistribution of their fiber mean, comprising a first peak at a firstfiber diameter of 2 μm or less, a second peak at a second fiber diameterof 4 μm or more, and a trough at a third fiber diameter between thefirst fiber diameter and the second fiber diameter, wherein 5% or lessof the polymeric fibers have a fiber diameter within ±0.5 μm of thethird fiber diameter, based on the total number of polymeric fibers. 10.The nonwoven web of claim 1, wherein the polymeric fibers in said layerare melt-blown fibers.
 11. The nonwoven web of claim 1, wherein thepolymeric fibers in said layer comprises 0.5-10 wt. % liquid crystallinepolymer.
 12. The nonwoven web of claim 1, wherein the polymeric fibersin said layer comprises polybutylene terephthalate.
 13. The nonwoven webof claim 1, wherein the nonwoven web has an air permeability of 20 cfmor more.
 14. The nonwoven web of claim 1, wherein the nonwoven web hasan air permeability of 40 cfm or more.
 15. The nonwoven web of claim 1,wherein the nonwoven web has a filtration ratio of 40% or more atparticle size of 10 μm.
 16. The nonwoven web of claim 1, wherein thenonwoven web has a filtration ratio of 60% or more at particle size of10 μm.
 17. The nonwoven web of claim 1, wherein the nonwoven web has afiltration capacity of 50 mg/in² or more.
 18. The nonwoven web of claim1, wherein the nonwoven web has a filtration capacity of 130 mg/in² ormore.
 19. A method for making the nonwoven web of claim 1, comprisingmelt-blowing a polymer mixture comprising at least two immiscible orpartially miscible polymers.
 20. The method of claim 19, wherein thepolymer mixture is obtained by compounding the two immiscible orpartially miscible polymers.
 21. The method of claim 19, wherein thepolymer mixture is obtained by physically blending the two immiscible orpartially miscible polymers.
 22. The method of claim 19, wherein thepolymer mixture comprises (a) 0.5-30 wt. % liquid crystalline polymerand (b) polybutylene terephthalate.
 23. A nonwoven web comprising alayer of polymeric fibers, wherein, based on a total number of polymericfibers, at least 25% of the polymeric fibers in said layer are coarsefibers having a fiber diameter of 3 μm or more, and at least 25% of thepolymeric fibers in said layer are fine fibers having a fiber diameterof 1 μm or less, and wherein the polymeric fibers in said layer have abimodal distribution of their fiber diameters, comprising a first peakat a first fiber diameter of 1 82 m or less, a second peak at a secondfiber diameter of 3 or more, and a trough at a third fiber diameterbetween the first fiber diameter and the second fiber diameter, wherein10% or less of the polymeric fibers have a fiber diameter within ±0.5 μmof the third fiber diameter, based on the total number of polymericfibers.
 24. (canceled)
 25. (canceled)
 26. A nonwoven web comprising alayer of polymeric fibers, wherein, based on the total number ofpolymeric fibers, at least 25% of the polymeric fibers in said layer arecoarse fibers having a fiber diameter of 5 μm or more, and at least 25%of the polymeric fibers in said layer are fine fibers having a fiberdiameter of 3 μm or less, and wherein the polymeric fibers in said layerhave a bimodal distribution of their fiber diameters, comprising a firstpeak at a first fiber diameter of 3 μm or less, a second peak at asecond fiber diameter of 5 μm or more, and a trough at a third fiberdiameter between the first fiber diameter and the second fiber diameter,wherein 10% or less of the polymeric fibers have a fiber diameter within±0.5 μm of the third fiber diameter, based on the total number ofpolymeric fibers.
 27. (canceled)
 28. (canceled)
 29. The nonwoven web ofclaim 1, wherein said layer of polymeric fibers is obtained by a methodcomprising melt-blowing a polymer mixture comprising (a) 0.5-30 wt. %liquid crystalline polymer and (b) polybutylene terephthalate.